STRmix™ collaborative exercise on DNA mixture interpretation

Abstract An intra and inter-laboratory study using the probabilistic genotyping (PG) software STRmix™ is reported. Two complex mixtures from the PROVEDIt set, analysed on an Applied Biosystems™ 3500 Series Genetic Analyzer, were selected. 174 participants responded. For Sample 1 (low template, in the order of 200 rfu for major contributors) five participants described the comparison as inconclusive with respect to the POI or excluded him. Where LR s were assigned, the point estimates ranging from 2 × 10 4 to 8 × 10 6 . For Sample 2 (in the order of 2000 rfu for major contributors), LR s ranged from 2 × 10 28 to 2 × 10 29 . Where LR s were calculated, the differences between participants can be attributed to (from largest to smallest impact): • varying number of contributors ( NoC ), • the exclusion of some loci within the interpretation, • differences in local CE data analysis methods leading to variation in the peaks present and their heights in the input files used, • and run-to-run variation due to the random sampling inherent to all MCMC-based methods. This study demonstrates a high level of repeatability and reproducibility among the participants. For those results that differed from the mode, the differences in LR were almost always minor or conservative.